1. Field of the Invention
This invention relates generally to a method and apparatus for packaging sensors and more particularly to chemical sensors, such as pH sensors, where a microelectronic substrate, such as an ion sensitive field effect transistor (ISFET), is integrally packaged with a counter electrode.
2. Discussion of the Related Art
Various ion sensitive field effect transistors (ISFETs), or microelectronic ion sensors, are known in the art. Such ISFETs have advantages for use as pH sensors such as being solid state, small size and relatively inexpensive to produce.
While semiconductor technology affords the opportunity of fabricating small sensors, reduced physical size introduces significant packaging challenges. An ISFET die includes multiple conductors, which may be routed to external electronic components. Traditional semiconductor packaging design employs electrical contact structures, such as wire bonds, that are made on the same side of the die as the chemical sensing ISFET. Because the sensing ISFET is wetted by the measurement sample, it is critically important to isolate these ISFET electrical contacts from the test liquid sample, particularly when ISFET sensors are operated over a broad range of temperatures and pressures. A first step in obtaining package integrity is to locate contact regions on the ISFET die backside as taught by Baxter in U.S. Pat. No. 4,505,799. While this is an important first step, silicon possesses uniquely different chemical and physical properties, such as a low value of thermal expansion coefficient in comparison with encapsulation polymer materials, which renders it difficult to develop and maintain isolation to process sample over sensor lifetime.
Additional techniques to enhance integrity, in the immediate vicinity of the ISFET die, are described in U.S. Pat. No. 5,068,205. In this known technique, shown in FIG. 1A, a glass header 12 has been utilized wherein the silicon die (ISFET) 17 is adhered to a first side 14 of a borosilicate glass carrier 16 over a through-hole 15 therein. The carrier 16 has a through-hole 15 in it to maintain uncovered the contact areas of the ISFET 17. The carrier 16 also has leads, collectively 18, on the second side 20 thereof to provide electrical access to the ISFET area from the edges of the carrier. The ISFET substrate 17 is electrostatically bonded to the glass carrier 14. Lead wires, collectively 22, are then bonded between the ISFET and the glass carrier leads. The glass carrier leads 18 and back of the ISFET 17 are then covered with an insulating cover 24 for protection. As shown in FIG. 1B, this header assembly 12 is then connected to a flexible circuit 26 for leading out through the probe body 28. This header 12 and circuit 26 assembly are then enclosed within the probe body 28 along with a "J"-shaped Hastelloy counter electrode 27, as detailed in U.S. Pat. No. 4,851,104, and potted with a thermoset polymer to isolate the internal components from the typically corrosive liquids of the sensing environment.
Certain other problems arise in utilizing the ISFET as a practical solution for low-cost sensing applications. Among these is the encapsulation of the device in a body or housing suitable for utilizing the ISFET as part of an ion-sensitive probe for commercial purposes. Typically, the ISFET illustrated in FIG. 1A is potted in a thermoset polymer so that the sensor electronics are not subjected to the often severe environment of the liquid being tested. Effective thermoset polymer encapsulation involves sophisticated assembly processes to obviate voids and to prevent coating of the active ISFET surface. These processes are constrained by the working life of the uncured thermoset polymer. On completion of the filling operation, thermoset polymers typically require additional time for the material to cure.
These aforementioned ISFET sensors are particularly useful when employed in potentiometric electrochemical measurement systems as probes in making pH measurements in industrial environments. In many instances earth-grounded solutions are subject to noise pickup due primarily to parasitic leakage currents flowing from the grounded solution through the measuring electrodes, the associated instrument or analyzer and through the analyzer power supply to the instrument ground. In cases where AC and DC voltages exist between the solution and instrument grounds, currents can be expected to flow via the lowest impedance pathway. This path usually involves unwanted current flow through the measurement liquid sample and the electrodes' lowest impedance path, which typically is the reference electrode. These problems are specially egregious in measurement samples of high purity water of 25.degree. conductivity values of 10 .mu.Siemens/cm or less. These spurious currents offset or shift the pH reading and cause drift in the sensor output with a commensurate drift in the measurement system accuracy. In order to offset and minimize these spurious currents and their undesirable effects, an additional electrically conductive electrode, or counter electrode, is inserted into the solution being measured in order to channel the spurious currents through this lower impedance electrode rather than through the reference electrode. The counter electrode is usually constructed of a electrically conductive material that is connected to the measurement system electronics and serves the function of the metallized gate in a metal oxide field-effect transistor (MOSFET); namely, it is the primary electrode to enable FET drain voltage and/or drain current control. A better understanding of the counter electrode's function within a potentiometric electrochemical measurement systems may be had by reference to U.S. Pat. No. 4,851,104 to Connery et al.
While the counter electrode technology offers sensor performance benefits, the use of a metallic or alloy material for the counter electrode would provide a location of possible liquid intrusion into the sensor, causing electrical leakage between internal electrode conductors, resulting in sensor malfunction. This intrusion is primarily due to the significantly different physical properties between the counter electrode and the housing and the dissimilar thermal expansion coefficients between these materials.
Design techniques to achieve sensor package integrity entail employing layered levels of protection to provide isolation of sensor conductors and sample fluid. These include backside contact, an electrostatically bonded intermediate structure followed by potting into a sensor subassembly. While this design technique provides for package integrity, it is complex, resulting in assembly costs which are in direct proportion to design and processing complexity.
Hence, there is need for an ion-sensitive microelectronic sensor package which is easily and inexpensively contained in an impervious housing while permitting media access to the ISFET sensor by effectively sealing the probe electronics from the media environment. Additionally, there is a need for packaging techniques that integrate a counter electrode in the sensor housing while eliminating the drawbacks of thermoset encapsulation.
Certain techniques for encapsulating piezoresistive pressure transducers with a conductive elastomeric seal are detailed in U.S. Pat. No. 5,184,107 to Maurer. This patent details a low cost piezoresistive pressure transducer utilizing pre-molded elastomeric seals in which at least one seal is electrically conductive. A piezoresistive, stress-sensitive element in the form of a diaphragm of semiconductor material having a thickened rim is held at its rim between a pair of pre-molded elastomeric seals in a two-piece housing. Electrical connections with external circuitry are made by conductive paths through one of the elastomeric seals, which makes contact with electrical leads that pass through the housing wall.